Present optical methods for determining concentrations of substances in aqueous solution are typically based on spectroscopic methods applying near-infrared (NIR) light, such as Fourier transform IR (FTIR) spectroscopy.
NIR light has the advantage that many substances have a characteristic fingerprint in this spectral range, thereby allowing a precise identification of substances. However, this advantages turns into a disadvantage when more complicated mixtures comprising many substances and possibly suspended particles. Overlapping spectral fingerprints and/or the light scattering by particles makes is very difficult to perform quantitative estimates of the content of substances.
The dielectric characteristics of aqueous solutions in the THz range have been studied in e.g.:                Hideaki Kitahara et al, “Dielectric characteristics of water solutions of ethanol in the terahertz region”, J. Korean Phys. Soc. 46, 82-85 (2005). This paper presents the THz spectrum of water-ethanol mixtures, measured in a transmission setup.        A. Dobroiu et al, “Monolithic Fabry-Perot resonator for the measurement of optical constants in the terahertz range”, Appl. Phys. Lett. 86, 261107 (2005). This paper describes how a prism with one facet being used in total internal reflection (TIR) geometry forms a THz Fabry-Perot resonator. A factor T is introduced as a correction factor in an expression for relative amplitudes of consecutive harmonics of the resonator. The factor T expresses the increased loss when a liquid sample is provided on the TIR facet. By measuring the difference in relative amplitudes of consecutive harmonics with and without sample, T can be measured. Comparing the obtained T-values with T-values obtained for samples with known concentrations of a substance allows estimation of a concentration in an unknown sample. The T-value is inherently a mean value over a narrow frequency range, and does not contain spectral information.        
However, most mixtures of industrial interest, e.g. in the foodstuff industry, does not consist of a single substance in pure water, but are rather complicated mixtures of many substances of different phases such as emulsions or suspended particles, bubbles etc. It is a disadvantage of prior art optical methods for determining concentrations of substances in aqueous solution that they are not adapted for use on such mixtures.